Valuable hydrocarbon components such as ethane, ethylene, propane, propylene, and heavier hydrocarbon components are present in a variety of gas streams, such as natural gas streams, refinery off gas streams, coal seam gas streams, and the like. These components can also be present in other sources of hydrocarbons, such as coal, tar sands, and crude oil. The amount of valuable hydrocarbons varies with the feed source. Generally, it is desirable to recover hydrocarbons or natural gas liquids (NGL) from gas streams containing more than fifty percent ethane, carbon dioxide, methane and lighter components, such as nitrogen, carbon monoxide, hydrogen, and the like. Propane, propylene and heavier hydrocarbon components generally make up a small amount of the inlet gas feed stream.
Several prior art processes exist for the recovery of NGL from hydrocarbon gas streams, such as oil absorption, refrigerated oil absorption, and cryogenic processes to name a few. Because the cryogenic processes are generally more economical to operate and more environmentally friendly, current technology generally favors the use of cryogenic gas processes over oil or refrigerated oil absorption processes. In particular, the use of turboexpanders in cryogenic gas processing is preferred, such as described in U.S. Pat. No. 4,278,457 issued to Campbell, as shown in FIG. 1.
Turboexpander recovery processes that also utilize residue recycle streams are capable of obtaining high ethane recoveries (in excess of 95%), while recovering essentially 100% of C3+ components. Such processes, though impressive in achieving high recoveries, consume relatively large quantities of energy due to their compression requirements. In order to reduce energy consumption while still maintaining high recoveries, an additional source of reflux is needed. It would be advantageous for such a reflux stream to be lean in desirable components, such as ethane and heavier components, and be available at a high pressure.
In many cryogenic recovery processes, efficiency is lost because of the quality of the fractionation tower overhead stream, which results in a reflux stream containing a considerable amount of C2+ components. Because the reflux stream has a considerable amount of C2+ components, any flash after a control valve on the reflux stream will lead to some vapor formation. The resulting vapor will have some amount of C2+ components that will escape the fractionation step and be lost in the overhead stream and subsequently in the residue gas stream. Additionally, equilibrium is reached at the top stage of the fractionation tower that allows more ethane to escape with the overhead stream.
It has been taught to use an absorber to generate lean reflux streams, such as in U.S. Pat. No. 6,244,070 issued to Lee et al. As described in Lee, vapor leaving the inlet separator is split three ways. The first vapor stream is cooled and introduced at the bottom of the absorber column. The second vapor stream is condensed and subcooled and is then introduced at the top of the absorber. The absorber produces an overhead stream that is used as a lean reflux stream for the main fractionation tower. The third vapor stream is sent to the expander for pressure reduction and work extraction. An alternate embodiment proposed by Lee involves using a portion of a high-pressure residue gas stream as a top feed stream to the absorber. In this case, vapor exiting the cold separator is split two ways, with one stream being cooled and sent to the bottom of the absorber, while the other stream is sent to the expander. A part of the lean residue gas is condensed under pressure and sent as a top feed stream to the absorber column.
A need exists for an ethane recovery process that is capable of achieving a recovery efficiency of at least 96%, but with lower energy consumption compared to prior art processes, which would be less expensive to operate than many prior art processes. A need also exists for a process that can take advantage of temperature profiles within a process to reduce the amount of C2+ components that are lost in the residue gas streams.